Emissions systems for compression-ignition (e.g., diesel) engines typically monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM) such as ash and soot, and nitrogen oxides (NOx).
With regard to reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized by exhaust aftertreatment systems to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx conversion rates, allowing the combustion technologies to focus on power and efficiency.
SCR systems utilize a reductant delivery system to introduce a reductant into the exhaust stream upstream of the SCR catalyst. When the proper amount of reductant is available at the SCR catalyst under the conditions, the reductant is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if a deficient amount of reductant is introduced into the exhaust stream upstream of the SCR catalyst, the SCR system may be unable to convert enough NOx.
On-board diagnostics requirements, stipulated by the U.S. Environmental Protection Agency and the California Air Resources Board, require exhaust aftertreatment systems to be monitored on-board and inadequate performance be signaled to the vehicle operator. SCR conversion efficiency is one of the most challenging diagnostics because typical NOx sensors used for control and diagnostics are cross-sensitive to ammonia, which also leads to high warranty costs. The warranty costs generally pertain to false faults, fault isolation, and improper control of urea injection. Other on-board diagnostics are falsely triggered due to the inability of existing diagnostic techniques to adequately address SCR aging. In addition to these issues, measuring or estimating ammonia storage in the SCR catalyst is very difficult.